You don’t HAVE to have a website to be an affiliate marketer. I think it is really good to have a website as you will be able to reach a lot more people with indepth information.
If you are on Facebook, you can promote the product(s) you are selling by posting messages there. Better would be to post a photo of the product along with a short writeup. (And while you are there, LIKE all the companies FB pages that you would like to work with.)
If you are on Twitter, you can promote the products as well. Try and write tweets that are a comfortable fit for the product. When I promote something on Twitter, I always tell people this is a promo tweet. I either use #sponsored, or #ad right in the body of the tweet.
You can buy Google Adwords and set up ads; you can do the same on Facebook.
Commissions and Cookies:
Companies decide what percentage of the sale will be passed on to you. Some companies offer 4%, some 8%, some as high as 20%. Don’t automatically take the product that gives 20% commission. You will need to think about a target market for that particular product. What happens if you can only sell a few units?
Perhaps the companies that are offering 4% are doing it because they have a highly marketable product that people are already buying. Or perhaps they are offering 4% because they are cheap. Either way, do some research and thinking around the type of products you want to sell.
As for cookies, the longer duration they are the better it is, for you. Here is an example:
A company offers a 30 day cookie. You agree to the terms and get started on your promotion. You put a message up on Facebook, letting your friends know about the product. They click on your affiliate link and they are taken to the company website.
They look around for 10 minutes or so, then they leave. They leave without buying. However, if at ANY time during the next 30 days (the duration of the cookie) they return to that site…the cookie is till there. That means whatever they order within the 30 days, you get the commission for the sale.
The above is WHY you should take seriously the duration of the cookie.